1. Field of the Invention
This invention relates to the fabrication of thin film magnetic heads and particularly to a CMP planarization technique for controlling overcoat and pole tip recession of such a head relative to a slider surface.
2. Description of the Related Art
A hard disk drive (HDD) uses an encapsulated thin film magnetic read/write head, called a slider, to read and write data on a magnetic medium or storage disk. FIG. 1 is a schematic illustration of such a device as is used in the prior art and whose improvement is the object of the present invention. The slider (10) has a pre-patterned air-bearing surface (ABS) (20) and is mounted on a head gimbal assembly (HGA) (not shown) that is activated by a mechanism and control circuitry to position the slider at various positions along the magnetic tracks on the disk (not shown). As the disk is rapidly rotated by a spindle motor (not shown), hydrodynamic pressure causes air flow between the ABS of the slider and the surface of the disk. This flow lifts the slider so that it literally flies above the surface of the disk (at a “fly height”) on a layer of air. The edge of the slider into which the disk rotates is called its “leading edge (30),” the opposite edge, which contains the read/write head (40), is called the “trailing edge (50).” The read/write head is within the body of the slider and the small region labeled (40) is the area on the surface of the slider at which the active portions of the read/write head intersect the surface. The head itself is below the slider surface and is not shown. Contacts formed at the trailing edge (60) are used to make connections between the read/write head and external circuitry. The aerodynamics of the slider motion lifts the leading edge higher above the rotating disk surface than the trailing edge.
The HGA provides a flexible connection between the slider and a load beam (suspension) allowing the slider pitch and roll capability when fly height is achieved. A pre-load downward force applied by the suspension is used to counteract and control the hydrodynamic lifting force. The position above the disk at which the pre-load downward force and the hydrodynamic upward force are in equilibrium is the targeted fly height of the slider. When a predetermined rotational speed and targeted flying height are achieved, the writing and reading of data commences. As a consequence of higher linear and track densities, the flying height and thus the distance between the read/write head and the storage disk, must be extremely small to ensure accurate transfer of data.
The read and write heads (40) are fabricated as device arrays in wafer form on a ceramic AlTiC (aluminum titanium carbide) substrate (substantially all of (10)) using well known semiconductor deposition techniques such as electroplating, CVD (chemical vapor deposition) and photolithographic patterning and etching. The read sensor portion of the head is typically fabricated as a sequence of magnetoresistive thin films (called an MR sensor). The write head is typically formed as a single electrically conductive coil wound, in two layers, around a yoke. The yoke spans a leading and trailing pole. The coil is typically formed of Cu windings while the poles are formed of a high permeability magnetic material such as NiFe. During the writing of data on the disk, current flows through the coil in alternating directions, which generates a relatively significant amount of heat. This heating affects nearby regions including the poles themselves and the insulating overcoat, which is typically sputtered alumina (Al2O3). This heating causes the emerging tip of the pole, the shields above and below the pole and read sensor and the surrounding overcoat to expand and protrude above the ABS plane in the direction of the disk surface. This expansion is referred to as thermal pole tip protrusion and overcoat protrusion. In FIG. 1, the pole protrusion would be evidenced within the ABS above region (40), just above the read/write head.
During the write operation, thermal protrusion causes the balanced-force distance between the lowest point on the slider ABS and the disk to be significantly less than the preferred flying height. With flying heights being already extremely low to enable accurate resolution of the narrow disk tracks, thermal protrusion can eventually result in a physical contact between the disk surface and the ABS plane. This contact can move the slider off its target track, can cause damage to the slider assembly, can damage the disk surface and/or cause a permanent loss of data.
The initial step in slider fabrication is the slicing of a wafer into pre-patterned blocks or “quads,” (quadrants) which are then further sliced into individual rows containing a horizontal array of read and write sliders. After this cutting is completed, the ABS of the row is polished by lapping to obtain critical dimensional control of the read and write elements as well as for the improvement of the surface finish.
A heating step is then applied to the rows of sliders using a convection oven to induce a thermal protrusion of the overcoat and pole tips. The amount of protrusion is a function of the time duration of the heating process (typically on the order of several hours) and the temperatures that are reached, which are typically in the 100° to 200° C. range.
Following the heating step, a process to control the profile of the ABS surface along its length (crown) and width (camber) is applied to the entire row of sliders. The shape profile of crown and camber are pointed out in FIG. 1. A thin film, diamond-like coating is deposited onto the ABS following this contouring procedure. Using photolithography and ion-beam plasma (IBE) or reactive-ion etching (RIE), the ABS is patterned with an advance air bearing (AAB) design (80) for flying height purposes. The typical final step of slider fabrication, after AAB patterning, is the dicing up of the row to form individual sliders.
As has already been noted, thermal protrusion is a result of the differences in the coefficients of thermal expansion (CTE) of the various materials used in fabricating the slider, the read sensor and the write element. The list below indicates the CTE, in units of 10−6/K, for the most important materials:    AlTiC: 7.5    Al2O3: 8    NiFe: 12    Photoresist: 90    Cu: 16.5
The AlTiC makes up the bulk of the slider body as shown in FIG. 1. Sputtered Al2O3, formed to a thickness of approximately 0.035 mm, forms a nearly transparent insulating coating on the ABS surface over the read and write sensors (located behind the circular region (40) and protruding upward through the ABS). The square regions (60), on the trailing edge of the slider, represent bonding pads for making electrical connections to the sensor elements. The NiFe is a ferromagnetic alloy used to form the pole tips and yoke elements. Photoresist, when hardened by baking, is used to separate coil layers within the yoke and is also used as a base layer on which to form the coils. Cu is the material used to form the coil windings.
It would be extremely advantageous if thermal protrusion could be eliminated during the actual use of the slider within the HDD by a compensating process that occurred during the fabrication of the slider. Several methods taught in the prior art attempt to achieve this object. Abels et al. (U.S. Pat. No. 6,428,715), teaches a method whereby after the lapping of the slider ABS using known techniques, the trailing edge of the slider base is contacted by an aqueous base having a pH of between about 9 and 13. Preferably the pH is between about 10 and 11 and most preferably about 10.6. The base solution is also preferably buffered and is a aqueous solution of an alkali salt of a weak acid such as an alkali carbonate/bicarbonate such as potassium carbonate and potassium bicarbonate or a mixture of sodium borate and sodium hydroxide. The solution also includes a suitable surfactant. The slider or the row of sliders is immersed in a filtered aqueous solution at a temperature between about 15-45 degrees C. for between 5 and 15 minutes, during which time an amount of the alumina overcoat is removed. This process removes the alumina overcoat protrusion without affecting the performance of the magnetic layers or unacceptably etching other portions of the slider.
Biskeborn (U.S. Pat. No. 6,712,985) teaches a method and apparatus for lapping a slider, utilizing a compliant pad covered with a lapping medium and a linearly vibratory lapping process. The lapping medium is a conventional medium (slurry of mechanical agents) to which a dilute acid has been added. The added acidity selectively removes portions of the iron-containing poletips to lower them beneath the ABS of the slider and, thereby, compensate for their thermal protrusion. Within the description of the method, the use of rotating lapping plates in the prior art is discussed and it is noted that they produce problems leading to degraded head performance.
The method taught in the present invention has distinct advantages over the prior art cited above. The objects of the present method and the means of achieving those objects will now be presented.